The present invention relates to a ceramic filter which is used for removal of the suspended matter, microbes, dust etc. contained in a fluid such as liquid, gas or the like.
Ceramic filters using a ceramic porous body have various advantages; that is, they are superior to polymeric membranes in mechanical strength and durability and accordingly high in reliability, they are highly anti-corrosive and accordingly low in deterioration when washed with a chemical (e.g. acid and alkali) solution, further they can be precisely controlled in the average pore diameter which determines the filtration ability. Therefore, they are in use in extensive fields such as water treatment, gas treatment, medicine, food and the like, for filtration and removal of the suspended matter, microbes, dust, etc. contained in a fluid such as liquid, gas or the like.
In recent years, worldwide water shortage and problems caused by pathogenic microbes including Cryptosporidium and 0-157 have become serious. This has necessitated a water purifier enabling easy production of water which is sufficiently safe and has high quality. As a water purifier which can effectively remove, by a simple operation, the harmful substances (e.g. suspended matter and pathogenic microbes) contained in a liquid, ceramic filters are drawing attention.
In order to maintain a required filtration ability and an intra-filter high permeability of fluid, many of the ceramic filters have a structure constituted by:
a base body made of a ceramic porous body (average pore diameter: about 1 to several hundreds μm), and
filter membranes provided on the base body, made of a ceramic porous body having an average pore diameter (about 0.01 to 1.0 μm) smaller than that of the base body. There is widely used, for example, a structure constituted by:
a tubular or monolithic base body having partition walls made of a ceramic porous body, wherein cells each functioning as a passage of a fluid are defined by the partition walls, and
filter membranes provided on the partition walls.
In many cases, ceramic filters are used by, as shown in FIG. 2, being accommodated in a housing 22 in such a state that the outer surface of base body 12 and its end face SE are separated gas-tightly by the use of an elastic sealing material 20 (e.g. an O-ring). However, in a ceramic filter (e.g. a ceramic filer 30 shown in FIG. 2), having a structure wherein the end face of the ceramic porous body (having a relatively large average pore diameter) constituting the base body 12 is exposed, there is a fear that a to-be-treated fluid F enters the inside of the base body 12 from its end face and is discharged outside the outer surface of the base body 12 without permeating the filter membrane 14. In a ceramic filter having such a structure, there is a fear that not only intended filtration is impossible but also mixing of to-be-treated fluid F into treated fluid after permeation through filter membrane 14 invites contamination of the treated fluid after permeation.
In order to solve such a problem, there was proposed a ceramic filter 10 such as shown in FIG. 1, provided with a glass seal 16 so as to cover at least the end face SE of base body 12 (see, for example, Patent Literatures 1 and 2).
In a ceramic filter having such a structure, since the end face of base body 12 is covered with a glass seal 16, there can be avoided a phenomenon that a to-be-treated fluid F enters the inside of the base body 12 from its end face and is discharged outside the outer surface of the base body 12 without permeating the filter membrane 14. As a result, the to-be-treated fluid F always permeates the filter membrane 14 and the partition wall 24 and is discharged outside the outer surface of the base body 12, whereby intended filtration becomes possible and the mixing of to-be-treated fluid F into treated fluid after permeation through filter membrane 14 and resultant contamination of the treated fluid after permeation can be prevented effectively.
Patent Literature 1: JP-B-1995-92527
Patent Literature 2: JP-A-2002-109923
With the above-mentioned ceramic filter provided with a glass seal, it is known that, when it is used continuously, the fluid permeability is reduced gradually and the ability as a filter is reduced. The reason is considered to be that the suspended matter (particularly, organic suspended matter) removed by filtration deposits gradually on the filter membranes of ceramic filter and the deposit plugs the pores of the ceramic porous body constituting each filter membrane. Hence, there is generally conducted, periodically or when necessary, a washing operation called “chemical washing” to the ceramic filter. There is conducted, for example, a washing operation which comprises removing organic suspended matter using an alkaline chemical solution (e.g. an aqueous solution of sodium hypochlorite) and then removing inorganic suspended matter using an acidic chemical solution (e.g. an aqueous citric acid solution). This chemical washing can peel off and remove the suspended matter, etc. which have deposited on the filter membranes of ceramic filter, whereby the fluid permeability of ceramic filter can be recovered to a level close to the original level.
In the membrane separator described in the Patent Literature 1, however, the base body and filter membranes in the vicinity of the glass seal has tended to be eroded when the above-mentioned chemical washing is repeated, and its corrosion resistance has not been sufficient.
Meanwhile, in the ceramic membrane filter described in the Patent Literature 2 proposed by the present applicant, there is controlled, in a given range, the difference between the alkali component content of the sintering aid used in formation of base body and filter membranes and the alkali component content in glass seal, whereby the erosion of the base body and filter membranes in the vicinity of glass seal is suppressed and the ceramic membrane filter has high corrosion resistance. Recently, however, there has been desired a ceramic filter which can withstand chemical washing of a large number of times and accordingly has excellent corrosion resistance; therefore, even with the ceramic membrane filter described in the Patent Literature 2, the corrosion resistance is not fully satisfactory and there is a room for improvement.
As described above, currently there is no disclosure on a ceramic filter which can effectively prevent the erosion of the base body and filter membranes in the vicinity of glass seal, can withstand chemical washing of a large number of times and has excellent corrosion resistance. Development of such a ceramic filer is solicited in related industries.